German retail sales and European unemployment could affect market sentiment early in the session, but the first big U.S. release is ADP's private-sector payroll report at 8:15 a.m. ET.

Economists expect that 175,000 jobs were added in July, down from 188,000 the previous month. Traders will probably view the reading as an early indicator of the key non-farm payroll report from the government on Friday morning.

Second-quarter gross domestic product follows at 8:30 a.m. ET. It is expected to show growth slowing to 1.1 percent from 1.8 percent in the previous period.

The next event is the Chicago Purchasing Managers Index at 9:45 a.m. ET. Economists forecast a reading of 51.5, down from 51.6 in June.

The Federal Reserve's interest-rate decision and policy statement at 2 p.m. ET are perhaps the most important event of the session. Attention will focus closely on how aggressively the central bank plans to taper quantitative easing.

Earnings will be released in the morning by companies including Anheuser-Busch Inbev, Comcast, Humana, Huntsman, Hospira, MasterCard, and Phillips 66. CBS, Ctrip.com, Lam Research, Marriott, Williams, and Yelp follow after the closing bell.

optionMONSTER® provides stock market insight, advanced options education, and actionable trade ideas to meet the needs of do-it-yourself investors. After spending decades in the trading pits of Chicago, Jon 'DRJ' and Pete Najarian founded the company in 2005 to help people better manage their own investment portfolios.